The performance of a paper machine, such as a tissue machine, is often limited by its drying section. Several factors in the drying section may cause a rupture in the web and the web produced may then have to be rejected due to unsatisfactory quality. The main factors affecting the efficiency of the machine's drying section are loss of time when there is no paper on the reeling drum and the quantity of paper that must be rejected after a rupture. In most machines which are run at high speed, the paper reel is ejected in the event of a rupture since it is difficult to rewrap a half-sized reel to continue reeling, and if it is too small the reel is rejected. One cause of web rupture is that when a web is running with no support between two consecutive sections in a paper machine, air currents will often cause turbulence and web flutter which sometimes results in the web breaking. Such air currents may come from the machinery, the layer of air at the boundary and the rotating drums in the paper machine. A section of the web draw in which the web runs without support is termed an "open draw".
In order to reduce web breakage it is known to stabilize or support the web by means of flutter suppressers along its draw. Such flutter suppressers may be in the form of plates or wings extending across the width of the predetermined web draw. When a web is to run along and slightly spaced from a flutter suppresser in the form of a plate, the boundary layer of air between the flutter suppresser and the web will be reduced and maintained. The tendency of the web to flutter will therefore decrease. Web stabilizers or flutter suppressing wings are described in U.S. Pat. No. 4,321,107 (Page) and U.S. Pat. No. 3,650,043 (Overly et al.). A section of the web draw where the web is supported is termed a "closed draw".
In the drying section of a paper machine the web usually passes a measuring station with a scanner for measuring properties such as moisture and basis weight. Scanners are described for instance in U.S. Pat. No. 5,164,048 (Bossen et al.) and U.S. Pat. No. 5,233,195 (Hellstrom et al.). The above-mentioned U.S. Pat. No. 4,321,107 states that a tissue-paper web runs from a Yankee-cylinder past a suppressing wing, through a scanner and then past a second suppressing wing. According to this patent specification the scanner forms an intermediate element between the suppressing wings. The part of the web draw where scanning occurs will therefore constitute an open draw. If a scanner is to be applied on a part of the web draw, scanning may not of course be impeded by any object such as a suppressing wing. The part of the web draw where the web passes a scanner will, however, have a greater tendency to web flutter and web rupture than the parts of the web draw where the web can be supported by a suppressing wing or similar element. The distance the web travels during passage of the measuring frame of the measuring station is of course relatively short, but it is still sufficiently long to be able to cause undesired flutter. Another drawback with the web flutter caused by the open draw is that measurement of the web properties will be less accurate.